The present invention relates to an integrated circuit semiconductor device having an improved wiring structure and more particularly, to a power supply wiring for a gate array type integrated circuit device.
In the gate array technique, various basic circuits such as TTL output buffer circuits and ECL output buffer circuits are fabricated in advance on a master substrate and "personalization wirings" or "application specific wirings" are made by interconnecting selected basic circuits and connecting power supply lines to selected basic circuits to obtain a desired function for a specific application.
The power supply lines are generally made of a wiring layer which should have a thickness as large as possible in order to reduce a resistivity thereof and decrease an unfavorable level shift in the wiring layer. However, the thickness of a single conductive layer such as aluminum layer has a limit, for example, of 2 .mu.m or less because of fine patterning, uniformity of the thickness and reliable maintenance of manufacturing apparatus such as vacuum evaporation apparatus by which the layer is formed. It has been accordingly practised to form the power supply line or a signal line of a low resistivity with double conductive layers, a lower layer and an upper layer superposed thereon via insulating film and electrically connected to the lower layer through a plurality of contact holes formed in the insulating film. The two layers have the same pattern or shape in a plan view.
The double-layer low-resistivity power supply or signal line requires at least three masks for formation thereof on the integrated circuit substrate, a first mask for the lower-level layer, a second mask for the contact holes and a third mask for the upper-level layer. A different double-layer line requires different three masks. In other words, three masks must be newly designed and produced when a pattern of the double-layer line is changed. This is disadvantageous, particularly in the gate array device in which a large number of masks are fixed and only such masks are changed in accordance with a function required for the specific application that are necessary to form the personalization wirings.